1. Field of the Invention
The invention involves a process for purifying liquids.
2. Description of the Prior Art
A large variety of processes are used for purifying liquids. Best known is simple distillation using a fractionating column. When the liquid is reasonably stable under ambient temperature conditions, as for example, in the case of water, petroleum products, etc., this process is not only satisfactory but often economically advantageous.
Many liquids cannot be purified to a high degree using ordinary distillation methods either because they decompose under conditions of distillation, or are azeotropes and not susceptible to separation by distillation methods. Alternative procedures are available such as the use of reduced pressures in distillation columns to reduce the boiling temperature of the liquid and the use of distillation columns made from such inert materials as Teflon and quartz. However, in many instances these alternative procedures do not give sufficient improvement for many applications. This is particularly true in situations where extreme purity is required. For example, in semiconductor processing and the production of glass fibers for optical waveguides, extremely pure products are often highly desirable, and in many cases required. Thus chemical reagents used in semiconductor processing and production of optical waveguides must exhibit levels of purity far beyond those usually obtained by conventional techniques. In such cases the distillation methods described above are often inadequate.
Of particular importance for chemical reagents used in semiconductor processing and production of optical waveguides is the removal of extremely small amounts of inorganic trace impurities. An additional problem, minor impurities can result from decomposition of the chemical reagent being purified. Purity levels of the order of less than several parts per billion are often desirable at least for certain elements. For example, in semiconductor processing where large amounts of chemical reagents are used in surface treatments, even very low concentrations of certain impurities (especially those which adversely affect the doping of the semiconductor) are highly undesirable. In optical fibers where extremely low loss is desirable, high purity chemical reagents are also important. For these reasons, a purification procedure which exhibits high efficiency under conditions of minimum decomposition is highly desirable. A particular example is hydrogen peroxide in aqueous solution which is extensively used in semiconductor processing as an oxidizing agent. Since this chemical agent may be used in large amounts on the surface of semiconductors, high purity is necessary to prevent unintentional doping and/or surface contamination of the semiconductor.